As the hot dip coated steel sheets comprising a zinc-based coating, there are known, in addition to the ordinary ones whose coating contains Al added in a very small amount for the purpose of inhibiting the growth of an alloy layer in the coating, those in which an element for improving corrosion resistance is added thereto, for example, those in which the coating comprises 4 to 53% of Al, 0 to 0.5% of Mg, 0.002 to 0.3% of Pb and the balance of Zn and inevitable impurities. As the hot dip coated steel sheets, coated with zinc/aluminum-based alloy, there are known those in which the coating comprises 0.1 to 0.3% of Al, 0 to 0.5% of Mg, 0 to 10% of Si, 0.002 to 0.3% of Pb and the balance of Zn and inevitable impurities. These hot dip coated steel sheets are usually continuously produced through the steps of heating reduction of a steel belt, dipping of the belt into a hot-dip metal bath and regulation of the coverage. However, in order to improve appearance, flatness, formability, etc., it is often carried out that spangles are minimized by spraying water on the coating while that formed is still in the unsolidified state immediately after the step of regulating the coverage, or that various mechanical treatment processings, for example, buffing, levelling and skin pass rolling treatments are applied after the coating has been solidified.
However, of those in which the spangles are minimized, or, even in the case of regular spangles, those which are obtained by processing in a hot-dip metal bath of zinc-based alloy or a zinc/aluminum-based alloy with additional elements such as Mg, or those in which mechanical treatment processings are further applied thereto to make chromate treatment for the purpose of improving the corrosion resistance or the paint film adherence, the surfaces are subject to color change into grayish black (hereinafter referred to as "blackening") in a relatively short period of several months even stored under ordinary conditions, thereby losing the appearance of silver white.
This blackening is presumed to be caused by changes in the state of elemental distribution or the crystal orientation on the surface of a coating, due to the treatment for minimizing spangles or the mechanical treatment processings, to form a film (an oxide layer mainly composed of Al, Cr and Zn) which is liable to absorb light, said film having a blacky appearance. This blackening becomes extreme particularly when a chromate treatment has been applied. Therefore, in order to lessen or make quiet the blackening, a post-treatment may be carried out in a non-chromate system. However, such post-treatment in the non-chromate system results in high cost for the treatment.
On the other hand, as a process which can prevent the blackening even when the chromate treatment has been applied for the post-treatment, there is known a process in which a coating is treated, after the mechanical treatment processings and before the chromate treatment, with an acidic aqueous solution of pH 1 to 4 or an alkaline aqueous solution of pH 11 to 13.5 containing Ni ions or Co ions or ions of both of these (as disclosed in Japanese unexamined patent publication No. 177381/1984). According to this process, although more inexpensive than the post-treatment of the non-chromate system, it is required to provide an additional treatment tank anterior to a chromate treatment tank, and also, since the treatment solution is strongly acidic or strongly alkaline, a washing tank is needed to wash out the solution. Thus, equipment cost becomes high and also the steps are made complicated.
Also, this process can prevent the blackening by deposition of Ni or Co on the surface of an active coating in the form of metals or oxides. However, since the deposition taking place here is effected by the displacement plating, the adhesion of the deposited metals or oxides is weak. Accordingly, the deposits are readily removed to cause the blackening on the removed face in the shape of spots, streaks or stripes, if slippage occurs between sheets when the steel sheet is wound into a coil, or when cut sheets are overlapped, handled or transported, or if friction is applied by roll forming, pressing or bending processings.
Further, if the aqueous solution, which is strongly acidic or strongly alkaline as mentioned above, is not sufficiently washed after the treatment, the remaining acid or alkali may be a factor for corrosion to cause the blackening or corrosion or the development of white rust on the contrary, or to cause the formation of local cells between Zn and the deposited Ni or Co, resulting in the microscopic corrosion.
This invention provides a hot dip coated steel sheet comprising a stable, blackening-preventive film toughly formed on the surface of a coating, and a process for producing the hot dip coated steel sheet, which makes it possible to form inexpensively and toughly the film on the surface of a coating.